Roentgen discovered X-radiation by the inadvertent exposure of a silver halide photographic element. In 1913, Eastman Kodak Company introduced its first product specifically intended to be exposed by X-radiation (X-rays). Silver halide radiographic films account for the overwhelming majority of medical diagnostic images. It was recognized almost immediately that the high energy ionizing X-rays are potentially harmful, and ways have been sought to avoid high levels of patient exposure. Radiographic films provide viewable silver images upon imagewise exposure followed by wet processing.
One approach, still in widespread practice is to coat a silver halide emulsion useful in radiographic films on both sides of the film support. Thus, the number of X-rays that can be absorbed and used for imaging are doubled, providing high sensitivity (that is, speed). Dual-coated radiographic films are sold by Eastman Kodak Company and other companies for various uses. Films that rely entirely on X-radiation absorption for image capture are referred to in the art as "direct" radiographic elements, while those that rely on fluorescent intensifying screens are referred to as "indirect" radiographic elements.
Direct radiographic elements have various uses, such as in industrial applications where intensifying screens cannot be used for some reason (for example, pipeline welds and turbine blades).
Another important application for direct radiographic elements is in dentistry where images of a patient's teeth and gums are made in order to provide desired diagnostic and preventive dental care. In dental diagnostic imaging a small piece of X-ray film (commonly referred to as a "chip") sealed in an opaque package is placed in a patient's mouth during X-ray exposure.
Due to the strongly penetrating nature of X-radiation, high quality direct radiographic elements (such as dental films) are generally comprised of a high coverage of silver on both sides of a flexible transparent film support. Various types of silver halide emulsions can be used in such films. Useful tabular grain silver halide emulsions for dental films are described in U.S. Pat. No. 5,370,977 (Zietlow).
Such films also generally contain one or more silver halide desensitizers to allow longer exposures of the high coverage, silver halide emulsions to safelights during handling and processing. Desensitizers are generally considered to be molecules having reduction potentials more positive than -0.9 volts versus a saturated Ag/AgCl electrode. Examples of desensitizers include dyes (for example cyanine and styryl dyes), nitro compounds and viologens. Electron-trapping dopants such as rhodium compounds and nitrosyl complexes of transition metal ions can also be used as silver halide desensitizers. Thus, desensitizers are useful for increasing safelight handling without affecting photographic speed for direct X-ray exposure.
A generally high silver coverage in high quality dental films minimizes patient and operator exposure to X-radiation by increasing photographic sensitivity. "High" silver coverage is meant to be generally from 12 to 22 g/m.sup.2. However, this high silver coverage also makes the films very sensitive to background radiation (radiation from terrestrial and cosmic sources) that is usually the main source of fogging before the films are even used. That is, the films have lower than desirable stability to storage fogging. For example, a commercial dental film marketed as KODAK EKTASPEED PLUS Dental Film by Eastman Kodak Company contains high silver halide coverage for improved photographic speed and image quality. It also contains a moderate amount of a silver halide desensitizer, Pinacryptol Yellow {6-ethoxy-1-methyl-2-[2-(3-nitrophenyl)ethenyl]quinolinium methyl sulfate} sufficient to improve safelight handleability but not enough to cause significant speed loss upon X-ray exposure. While this product has desirable photographic speed, there is a desire to provide improved speed without decreasing stability to background radiation.
Direct X-ray exposure films of lower cost and image quality can be made by providing reduced silver halide coverage in the silver halide emulsion layers.
Copending and commonly assigned U.S. Ser. No. 09/334,310 (filed Jun. 16, 1999 by Dickerson) describes "low silver" dental films having lower cost without undesirable loss in photographic speed. Silver halide "desensitizers" (for example, 6-chloro-4-nitrobenzotriazole) can be used in such films at low amounts because fog formation is reduced with the lower silver halide coverage. This application has been abandoned in favor of Continuation-in-part application U.S. Ser. No. 09/604,032 (filed Jun. 27, 2000 by Dickerson).
For these reasons it has been difficult to provide high quality dental films that simultaneously have high sensitometric speed, safelight handleability, low graininess and stability to fogging during storage. Formulating a film having all of these desirable properties is not simply mixing the components that increase each property since some of those components work in opposition. Thus, there continues to be a need in the art for such high quality direct radiographic films (especially for dental films) that have all of the noted properties.